This invention relates to a method and apparatus for determining the average clearance between closely fitting cylindrical parts and, more particularly, to a means for determining the electrical capacity between a piston and bore having a dielectric material therebetween, which piston and bore are randomly and unpredictably shorted.
During the manufacture of pistons and bores for devices such as diesel engine fuel injection pumps, the accurate measurement of the clearance between the cylindrical parts is critical. The accurate measurement of such clearances is difficult with fluid leak rates or pressure drop measurements being currently employed to infer clearances. Observe that because the mechanical area of the piston and bore, and the dielectric constant of a dielectric material positioned therebetween can be measured with requisite precision, the very small unknown diametral clearance between the piston and bore can then be accurately determined by measuring the electrical capacitance between the piston and bore and calculating the average spacing of the capacitor thereby realized.
While attempting said capacitance measurement, the piston and bore function as a capacitor only at random and unpredictable times because of minute roughness or imperfection in the mating surfaces that ordinarily puncture the dielectric material and cause the piston and bore to short together electrically. No devices known to applicant currently provide for the measuring of the capacity between the piston and bore during the short random and unpredictable time intervals in which they are not shorted.
Accurate measurement of the diametral clearance between the piston and bore is critical in quality assurance during manufacture of numerous products. Before assembling a device such as a diesel engine fuel injection pump, the clearances must be checked, preferably during the manufacturing process. The present invention also has application to certain types of hydraulic valves and cylinders requiring close tolerance fits as well as to firearms.